The Serial Port is a low-level way to connect to the Raspberry Pi.
The communication depends on byte wise data transmission and is generally available even before boot time.

First interaction with the board

Note that the Raspberry has no PREBUILT COM port. Soldering/protoyping and technical knowledge is required !

A UART is available in the Broadcom SoC, and its RxD and TxD lines are available on the GPIOs but you need a board or cable to convert 3.3V UART signals to RS-232. They do however connect directly to a USB serial legacy device, provided it uses 3V3 levels. If the device uses 5V levels you will need level converters (which could be simple resistor dividers, or could be specific IC's) See this tutorial for one example on how to build 3.3V to RS-232 levels converter with a breadboard, MAX3232CPE and five 0.1 uF capacitors. A 5V to 3V resistor divider can be built from a 2K2 (top) and a 3K3 (bottom) resistor.

Connect the serial cable to your COM circuitry , and connect the other end to the COM port or USB Serial Adapter in the computer.

The TxD and TXD signals can also be connected directly to similar signals on a microcontroller board like the arduino provided the signals are all at 3V3 levels. It's still a good idea to put 2K2 series resistors in the lines to prevent damage when two outputs are connected together, which could also happen if a GPIO input pin is accidentally programmed as output.

Serial Parameters

The following parameters are needed to connect to the Raspberry. All parameters except Port_Name and Speed are default values and may not need to be set.